Method in evaporation of waste liquor discharged from continuously operating cellulose digester or boiler



Feb. K8, i969 E. H. BACKTEMAN 3,428,107

METHOD IN EVAPORATION 0F WASTE LIQUOR DISCHARGED FROM CONTINUOUSLY OPERATING CELLULOSE DIGESTER 0R BOILER Filed Aug. 9. 1966 Sheet of 2 coNDENsER Feb. 18, f E H. BACKTEMAN 3,428,107

METHOD IN EVAPORATIION OF' WASTE LIQUOR DISCHARGED FROM K CONTINUOUSLY OPERATING GELLULOSE DIGESTER OR BOILER Filed ug. 9. 1965 Sheet 2 0f 2 ER/A/ H060 5A c/frfM/q A/ /NVE/VTO.

United States Patent O 3,428,107 METHOD IN EVAPORATION OF WASTE LIQUOR DISCHARGED FROM CONTINUOUSLY OPERAT- ING CELLULOSE DIGESTER OR BOILER Erik Hugo Backteman, Stockholm, Sweden, assignor to AB Rosenblads Patenten', Stockholm, Sweden, a Swedish company Filed Aug. 9, 1966, ser. No. 571,313 Claims priority, application Sweden, Aug. 19, 1965, 10,851/ 65 U.S. Cl. 159-47 1 Claim Int. Cl. B01d 1/26 ABSTRACT OF THE DISCLOSURE The method relates to evaporation of waste liquor or lye discharged from a continuously operating cellulose digester or boiler which operates at high pressure and at high temperature, i.e. the liquor being in the state in which it leaves the digester after completion of the pulping process. More particularly the method relates to the case, in which the liquor after pre-evaporation according to the flash-expansion principle will be evaporated further in an indirect evaporator of multiple effect type.

According to a known method for this purpose, the liquor, which for example can be obtained at a pressure of about 8 atmospheres superatmospheric pressure and at a temperature of about 170 C., will first be subjected to a preliminary evaporation by reduction of the pressure in two successive expansion stages, to the effect that in the first stage liquor vapor of a temperature of about 140 C. suitable `for alkalinization of the digester Chips will be obtained and utilized for this purpose. In the second expansion stage, from which the liquor or lye will be drawn off and stored in a tank for subsequent final evaporation, the pressure will be reduced further, so that the lye will get a temperature of for example about 100 C., which is considered to be a suitable temperature for storage as well as a suitable supply temperature for the final evaporation process, the lye vapor produced, having a corresponding temperature, being withdrawn through a condenser. The nal evaporation process is effected in an indirect multiple stage evaporator, which in its first stage is heated by freshly generated steam and in each one of its subsequent stages is heated by lye vapor from an immediately preceding heating stage.

In the first place the invention aims at effecting such cooperation between the pre-evaporation of the multi-ple expansion stage type and subsequent evaporation stages, that all vapor liberated in the pre-evaporation during the expansion stages following upon the first stage of evaporation will ybe utilized as heating vapor during subsequent evaporating operations in a most eicient manner in order to reduce the need for freshly generated steam, while the vapor from the first evaporation stage can be utilized separately, for example for the alkalinization mentioned and/ or for other purpose.

The invention is primarily characterized by the fact that the lye Ivapor or liquor vapor formed is conducted as an additional heating vapor from each expansion stage, with the exception of the first one, to the said heating stage according to the rule: from the second expansion stage to the second heating stage etc. in turn to the extent that there are any further expansion stages and ICC heating stages, the last expansion stage, which Supplies the pre-evaporated liquor to some of the heating stages of the multiple evaporator except the last one being adapted to supply additional heating vapor to the next following heating stage.

The invention will be more particularly disclosed with reference to the accompanying drawings; in which FIGS. l and 2 respectively each illustrate in the form of a flow diagram a modification of an evaporation installation for performing the method according to the invention, and

FIG. 3 illustrates part of the installation according to FIG. 2.

Identical parts in the different figures are provided with the same reference characters. All figures are schematic and are only intended to serve as examples `without restricting influence.

In FIG. 1 the liquor from the digester (not shown in the figure) flows through the intake conduit 5 and the connecting conduits 6, 7, 8 and 9 and through expansion vessels 1, 2, 3 and 4 in the sequence now stated.

I, II, III, IV and V are the different heating stages of an indirect multiple stage evaporator of substantially conventional design, through the liquor chambers of which the liquor flows from the last expansion vessel 4 in the sequence III, W, V, I and Il through the connecting conduits 9, 10, 11 and 12, the circulation pump 13, the connecting conduits 14 and 15, and the discharge conduit 16. Stage I will hereby be fed with freshly generated vapor or steam as heating medium through conduit 17, while the subsequent stages II, III, IV and V are supplied with liquor vapor as heating medium from the immediately next preceding stage through the vapor conduits 18, 1.9, 20 and 21 respectively; the liquor vapor from stage V being withdrawn to a condenser (not shown in the figure) through the conduit 22. As known, such an evaporator, the pressures in said conduits and the chambers communicating therewith will fall from one conduit to the next conduit in the sequence stated. Due to the fact, that the vapor chambers of the expansion vessels 2, 3 and 4 communicate with said vapor conduits 18, 19 and 20 respectively, through conduits 23, 24 and 25, respectively, corresponding differences will be maintained between the pressures in these vessels. The same result will be obtained, if the expansion vessels communicate directly with a chamber, communicating with the vapor conduits stated, for example the expansion vessel with the vapor heating chamber in stage II through conduit 23 or with the liquor vapor chamber of stage I etc.

As a result the hot liquor from the digester, on passing through the expansion vessels will supply vapor under stepwise reduction of pressure, this vapor serving as additional heating vapor and being discharged from the expansion stages 2, 3 and 4 through the conduits 23, 24 and 25 respectively, to the heating stage Il, to the heating stage III, and to the heating stage 1V, whereby the liquor thus pre-evaporated will be conducted through the connecting conduit 9 to the liquor chamber or space of heating stage III at least approximately at the pressure and temperature conditions prevailing in that chamber.

By a suitable degree of throttling in the liquor conduit 6 from the expansion vessel 1 the fall of pressure in the first expansion stage will be limited, so that the vapor there produced which is conducted through the conduit 26 to be utilized for the heat treatment of the digester chips, will get a suiciently high temperature. Any excess of such vapor for this purpose is conveyed through the lbranch conduit 26a as a heating medium to a preheater 27 for the liquor conveyed through the conduit 14. In this conduit also additional preheaters 28, 29 and 30 are arranged which are heated by condensate from the heating stages II, III and IV through conduits 31, 32 and 33 respectively. From a pure heating technics standpoint, one could of course imagine alternatively to convey such excess vapor as an additional heating vapor to heating stage I, for example to the fresh steam-conduit 17, but in this case the discharged condensate of the fresh steam, which should be utilized as feed water, would be contaminated. In this connection it should be pointed out that the expression fresh-steam should 'be understood -as relating to the vapor, which yields a condensate of suicient purity for this purpose, :disregarded from the fact from what source it is delivered, for example directly from a steam generator or in the formv of drawn olf steam from a turbine or any other installation.

In such an installation according to the invention the steam consumption will be about 150` kilos per 1000 kilos evaporated water, while a conventional -stage evaporator consumes approximately 250 kilos of steam for the same amount of evaporation. If combined with preevaporation in two stages of expansion and condensation of the thereby produced vapor in accordance with the known method indicated above, only a very modest improvement can be obtained, viz. a'bout 220 kilos per 1000 kilos of evaporated water. Thus, the improvement of the steam economy achieved by means of the invention is important.

The installation according to FdG. 2 differs from that according to FIG. 1 only insofar as the expansion vessels 2, 3 and 4 according to FIG. 1 have been replaced Iby expansion chambers 2a, 3a and 4a in the steam and liquid separators 35, 36 and 37 respectively of the heating stages I, I-I and III, most clearly illustrated in FIG. 3, so that the previously used steam conduits 23, 24 and 2S could be omitted.

By means of yan intermediary bottom 38 with steam piping 39 the separator, for example 35 in the heating stage lI, is separated into a separator chamber 40 below the bottom and the expansion chamber 2a above the same. In the separator chamber vapor and liquor from the liquor and vapor mixture produced during the beating stage are separated from each other, the liquor being withdrawn through the conduit and the vapor being discharged through the vapor piping 39, the expansion chamber 2a and the conduit 118 to the next heati-ng stage II. The liquor from the first expansion stage is introduced tangentially into the expansion chamber through the conduit 6, so as to be separated by the centrifugal force from the thereby produced vapor, and collected at the bottom 38 about the vapor piping 39, from where it is discharged through the conduit 7 to the next expansion chamber 3a. The vapor produced is simultaneously mixed with the through-flowing vapor from the separator chamber, and will 'be entrained to the next heating stage II as additional heating vapor. The processes are analogical in the two subsequent stages 3a, II and 4a, II I.

By this arrangement it will be evident that the apparatus required for performing the invention will not be substantially more complicated nor substantially more expensive than a conventional indirect multiple stage evaporator of the same evaporation capacity. This is of course a substantial advantage, although it should be understood, that t'he great gain in steam economy would permit considerable investments in installations without substantially reducing the practical value of this gain.

The liquid llow direction III-'lV-V-I--II illustrated, is most frequently used in S-stage evaporators, in first line because the liquor, on account of its viscosity increasing vwith the increasing concentration, preferably should be finally evaporated at a high temperature, which however should not be permitted to rise above about C., that is to say the highest temperature at which the liquor can be storaged at atmospheric pressure. Suc'h a temperature of the liquor will generally be found in stage II, which in consequence then should constitute the final stage, whereby the aforesaid liquor flow conduction will be the most convenient one. It is a generally accepted requirement, that the liquor should be fed to a multiple stage evaporator at a temperature, which at least not substantially is lower than the digesting teruperature in the heating stage to which it is supplied. Thus, the pre-evaporated liquor, fed to the heating stage III, should have 'a temperature in the proximity of the digesting temperature in this stage, which it will acquire in the last expansion vessel 4 because the latter, which supplies additional vapor to the next following heating stage IV, thereby also is subjected to a pressure approximately equal to the pressure prevailing in the liquor chamber of the heating stage III.

By suoh a liquor flow in a S-stage evaporator one can thus, as illustrated, while satisfying the aforesaid requirement, utilize four expansion stages within the scope of this invention.

The number may be increased to tive, if stage IV is permitted to constitute a supplying step, resulting in still further reduction of the steam consumption. A further increase of the number of stages does not result in any additional advantage, because an arrangement wherein stage V is a feeding step and a further expansion stage is included would cause that the vapor from the latter stage would ow to the discharge 22 directly. On the other hand, moving the intake to a stage operating at a higher temperature, would cause a reduction of the number of such expansion stages, for example to three stages on feeding in at stage II, whereby obviously also another heating stage has to constitute the final step.

Nor is the invention restricted to the case in which the vapor from the first expansion stage to some extent is utilized for steaming the chips but this vapor can also be used for other purposes. In sulphate digestion the vapor will for example contain turpentine, which should be recuperated. Then the vapor may conveniently be conducted in its entirety to a so called turpentine condenser. I-n this case the preheater 27 may possibly serve such purpose.

With the guidance of the examples illustrated and described any one skilled in the art will be able to adapt the invention to any other number of heating steps and different types of liquor flow.

What I claim is:

'1. A method in evaporation of waste liquor supplied at high pressure and high temperature from continuously operating cellulose digester or boiler, wherein said liquor is first pre-evaporated and cooled by being passed through a plurality of successive flash expansion stages from which the liquor vapor thereby released is discharged and from the last said flash expansion stage of which the pre-evaporated and cooled liquor is thereafter passed to an indirect multiple effect evaporator where it is further evaporated, each effect having a heating-evaporatng stage and a vapor separating stage and the number of effects is the same as the number of expansion stages and in which the lirst heating-evaporating stage is heated by fresh steam and each one of the succeeding heatingevaporating stages is heated -by liquor vapor from the next preceding vapor-separating stage, wherein the improvement comprises in combination the steps of passing from each of said flash expansion stages with the exception of the first one the liquor vapor there released as additional heating vapor to a corresponding one of said heating stages according to the rule: from the second expansion stage to the second heating stage from the third flash expansion stage to the third heating stage and continuing correspondingly until ultimately from the nth -flash expansion stage to the nth heating stage and passing the pre-evaporated liquor from said nth ash 6 expansion stage to the (n-1) heating stage where n FOREIGN PATENTS is the number of the flash expansion stages and heating 709,044 8/1931 France stages respectively.

References Cited UNITED STATES PATENTS I. SOFER, SSI'SLIH Examiner.

2,544,885 3/ 1951 Jacoby et al. 159-17 X Us, C1, X R 2,651,356 9/ 1953 Sadtler 159-20 3,021,265 2/ 1962 sadtler et al. 15s-2o X 10 159-20; 202-174 NORMAN YUDKOFF, Primary Examiner. 

